The invention relates to extracting a common bass signal from a multi-channel audio signal.
In earlier days home stereo systems typically included only two speakers, one for the left channel and another for the right channel. Generally, each of the speakers was designed to reproduce both bass information (e.g. &lt;200 Hz) and higher frequency information (&gt;200 Hz). This meant that each speaker had to have a large woofer for low frequencies, and one or more smaller speakers for the higher frequencies. In other words, speaker enclosures for high quality systems tended to be large because accurate bass reproduction required large woofers.
More recently, stereo system designers have come to appreciate that it is not necessary that all speakers in a sound system be capable of reproducing the bass sound information. Bass after all is relatively omnidirectional which means that it is difficult to determine where it was coming from. Thus, a number of stereo system designers have moved away from using a bass woofer in each speaker enclosure and have instead used a single, separately located subwoofer for the entire stereo system. In such systems, the bass information that is present in each of the two stereo signals is extracted, combined, and sent to the single subwoofer. By not requiring the other speakers to also handle bass, the larger, relatively expensive woofers can be eliminated and the speakers can be made much smaller and less expensive. The reduction in size makes possible a much less conspicuous installation when the sound system is installed in the home.
As home speaker systems have become more sophisticated so too have the recorded sound tracks that are available for playback at home. For example, sound tracks now include audio information for more than two channels. In addition to left and right channels, there may also be a center channel and a surround channel. The center channel is played through speakers that are located in front of the audience and midway between the left and right speakers. The surround channel is played through two sets of speakers located behind the audience and on either side of the room. Of course, since typical home entertainment systems are designed to receive or handle only a stereo signals, they do not have the capability to extract more than two channels of sound from the recorded media. Thus, to make the multi-channel sound tracks compatible with home entertainment systems, the sound tracks are combined or encoded in some way to produce two audio channel signals that contain sound information for all four channels.
A popular method for encoding four channels into two channels for home stereo systems is the Dolby.TM. surround sound encoding technique illustrated in FIG. 1. In that diagram, the blocks with summation symbol (i.e., .SIGMA.) represent circuits which add the inputs to produce a summation signal and the blocks with the phase angle symbol (i.e., .phi.) represent all pass networks which are characterized by an amplitude response that is flat over the relevant frequency range and a phase response that varies linearly with frequency (i.e., all frequencies are delayed by the same phase). The circuit generates left total and right total channel audio signals, L.sub.t and R.sub.t, as follows: EQU L.sub.t =L+0.707C+0.707 jS EQU R.sub.t =R+0.707C-0.707 jS,
where j=(-1).sup.1/2. That is, the surround channel signal appears in quadrature with the left, right, and center channel signals and the surround signal components of the left and right total channel signals are equal and 180.degree. out of phase with each other.
On the decoding side of the system (e.g. during playback), a center channel signal is produced from the stereo signal by combining the left and right total channel signals, i.e., EQU L.sub.t +R.sub.t =L+R+1.414C.
And a surround channel signal is produced by subtracting the left channel signal from the right channel signal, i.e., EQU L.sub.t -R.sub.t =L-R+1.414jS.
Though this techniques does not recover each of the four channel signals separately, the decoded signals that are generated by this technique produce a psychoacoustic effect that is similar to a true four channel surround sound when played back in a four or five speaker system.
Note, however, that it is not readily apparent how to combine the signals so that a single subwoofer can be used to reproduce the entire bass as is done in the above-mentioned stereo system. Since low bass frequencies can originate as left, right, center, or surround channel information, all bass information cannot be represented by a simple summation of L.sub.t +R.sub.t. Such a simple summation would cancel the bass information found in the surround signal. Another logical but equally unacceptable choice would be to combine the bass of the decoded center channel signal and the decoded surround channel. But notice what happens when this is done. The resulting signal is equal to: EQU (L.sub.t +R.sub.t)+(L.sub.t -R.sub.t)=2L+1.414C+1.414jS.
This produces destructive interference of the right channel information. Thus, if the bass signal is only present in the original right channel signal, it will not be reproduced in such a system.